libexpr: Support structured error classes

While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.

(cherry picked from commit c6a89c1a1659b31694c0fbcd21d78a6dd521c732)
Change-Id: Iced91ba4e00ca9e801518071fb43798936cbd05a

1 files changed, 2 insertions, 8 deletions

diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index b9ff01c16..261722d1b 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -38,17 +38,11 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.fetchMercurial");
             else
-                throw EvalError({
-                    .msg = hintfmt("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]),
-                    .errPos = state.positions[attr.pos]
-                });
+                state.error<EvalError>("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]).atPos(attr.pos).debugThrow();
         }
 
         if (url.empty())
-            throw EvalError({
-                .msg = hintfmt("'url' argument required"),
-                .errPos = state.positions[pos]
-            });
+            state.error<EvalError>("'url' argument required").atPos(pos).debugThrow();
 
     } else
         url = state.coerceToString(pos, *args[0], context,